We’re no longer just strangers in a crowd.

Imagine any street corner in any town where, let’s say, four people — Alexandria, Larry, Cory and Cameron — are lost in private thought.

Without a single conversation, without even knowing their names, we could learn that Alexandria’s angry ex-boyfriend posted her photo on a “revenge porn” website. That Larry is mourning the death of his daughter. That Cory is trying to scrub her image from friends’ social networks. That Cameron picked the wrong place to hide from police.

In each case, a simple photograph of the four strangers, combined with the power of data, opens the door to deeply personal details. That’s one of the many ways digital technologies are turning our once-personal lives into a global show-and-tell and redefining our expectations of privacy.

Almost every day brings new revelations about how Big Brother snoops on us and Big Data mines our online activities for profit. Even so, we are only beginning to understand the power of these incursions. In a few years, our faces alone, snapped on a street, in a crowd, or posted by a friend on the Internet, will be the key for a search engine to reveal the stories of our lives.

There may be nothing that technology is changing more dramatically than privacy. What is happening with our images online is just one example of our digital reality: We’re living life out loud — secrets and all.

To be sure, gossip is as old as our species. It spread through villages in whispers or over our grandparents’ “party line” phones. And the impulse to share photos of ourselves started the moment Louis Daguerre first fixed images onto sheets of silver-plated copper.

But information moved at a human pace — eventually forgiven, forgotten.

Now this information travels across continents with just a click. And it can remain virtually forever in the data stream. There is no “erase” key.

People like Alexandria discover that in new and shocking ways every day.

“I am flabbergasted. It is crazy,” she said after a reporter discovered a topless photo of her on a pornographic website that her ex-fiance had posted more than two years after their breakup.

He has remarried and has a child; she is engaged. Yet the photo persists SEmD and the Netherlands-based website demands $500 to remove it.

We’re not publishing her name to avoid a further invasion of her privacy. She had no idea her bare-chested image was out there — with her name, hometown and age attached — until the reporter called. “It was a personal thing between me and him,” she said. “It happened one time, and I didn’t think he’d hold onto them.”

(A similar California case led to criminal charges recently when the attorney general announced the arrest of a San Diego man accused of asking women for up to $350 each to remove illicit photos from another revenge site.)

Here is the simple math behind our brave new digital world, with its blurry boundaries between private and public: Photos (plus) name (equals) information.

Photos — a snapshot at a party or a “selfie” shot in the bedroom — can reveal names, using tools like Google Plus’ “Find My Face” and Facebook’s facial recognition software. And geo-tagged posts on social networks can reveal the precise location of your whereabouts.

Then our names become a pipeline to once-private information, such as home address, age, employment, taxes paid, political affiliations and campaign contributions.

Where is this headed? For better or worse, we’re becoming one vast Neighborhood Watch. And the surveillance doesn’t stop at front doors, but follows us inside.

We’re not just living with Big Brother peering over our electronic shoulders. We’re also a more tightly connected nation, with many fiercely committed busybodies.

It used to be expensive to make things public and cheap to keep them private, notes Internet scholar and New York University professor Clay Shirky. Now it’s expensive and time-consuming to make things private. But it’s cheap — and easy — to be public. About 2.5 billion people worldwide have a digital camera in their pockets, often attached to a smartphone, according to Hong Kong-based mobile technology consultant Tomi Ahonen. If each person snaps 150 photos a year, that adds up to a staggering 375 billion images annually.

San Francisco Bay Area resident Cory Colligan knows the challenge of protecting her privacy. When a friend tags her in a Facebook photo, she “untags” it, or politely emails a request to delete it. If feelings are hurt, she explains her concern to friends over lunch.

“I love social media, but I don’t post photos. I don’t want to be out there that way. Period. I’m a private person,” Colligan said. So she decorates her Facebook page with inspirational messages and landscapes; only her professional LinkedIn profile has a photo.

But sharing photos through social media will escalate, experts say, because it fills a human need for connection and intimacy. They help us stitch together our communities, keeping us in closer touch with the people we love.

“Tools like Flickr reverse the old order of group activity — transforming ‘gather, then share’ into ‘share, then gather,’ ” Shirky writes in “Here Comes Everybody,” his book about the Internet and group dynamics.

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